Rechargeable batteries are used in many applications to power a variety of devices. Different devices will discharge rechargeable batteries at different rates depending on the function being performed by the device and corresponding load being applied across the battery terminals. For example, a portable computer may discharge a rechargeable battery quickly when computing complex graphic calculations on a processor and rendering a graphic image on a display. The same portable computer may discharge the rechargeable battery more slowly when it is placed in “stand-by mode” and operation of the computer is temporarily suspended. Even when the portable computer is turned off, the rechargeable battery may also continue to discharge a small amount of current over time due to the internal resistance present in the battery.
Generally, the rechargeable battery is charged with a transformer that converts current from a conventional electrical outlet or automobile lighter into direct current suitable for charging the battery. Once the rechargeable battery reaches a maximum voltage, it is fully charged. To protect both the rechargeable battery and the electronic device that it powers, it is important to carefully monitor and control the charging and discharging processes. Specifically, a battery can overheat and be damaged during the charge cycle if it is charged beyond the specified battery capacity. Overcharging can also harm the electronic device as well as people handling the device if the battery leaks or is damaged. In the discharge cycle, for example, an electronic device may be damaged if a short develops within the battery or the device and the sudden increase in current causes the battery or device to overheat or melt.
The device used to measure the charge/discharge state of a battery is popularly called a “gas gauge.” Like the gas gauge on an automobile, the battery gas gauge measures how much charge is stored in a battery. Conventional gas gauge devices measure the current flow into and out of the rechargeable battery to measure the battery's charge. These conventional gas gauges detect the current flow using a fixed resistor coupled in series between the battery and the load. The voltage drop across the series resistor is directly proportional to the current flow measurements into or out of the rechargeable battery. Unfortunately, this series resistor, though typically very small in size, consumes a significant portion of the available power delivered by the rechargeable battery over time. Moreover, a small series resistor cannot be used to accurately detect the wide range of currents drawn by many of the electronic devices. That is, the voltage drop produced by the very small series resistor may only be accurately detected when the current flow is high. If the current flow is low, most conventional gas gauges may inaccurately measure the very small voltage drop across this very small resistor. For example, the conventional gas gauge may not accurately detect the lower current used when a computer is placed in “stand-by” mode. Although the series resistor size can be increased to increase measurement accuracy, the larger series resistor will also increase the power lost across the series resistor and, at high currents, further reduce the voltage available to drive the load.
Conventional gas gauges also have difficulty determining the battery charge when a battery is used over long periods of time. These gas gauge devices must keep an accurate time base to integrate the current charge and discharge over time and determine the remaining battery charge. Consequently, accurate battery charge measurement depends on how accurately a conventional gas gauge measures elapsed time over several days or, in some cases, several months of battery usage. Keeping an accurate time basis generally requires additional circuitry and added complexity in the design of the gas gauge.
Even if battery charge and other information related to charging a battery were available it is difficult to communicate these facts with other devices. The battery and charger typically cannot communicate with other devices because there are no standards for such communication. Further, even with communication standards provided, they are difficult and expensive to implement for typical applications.